Ngwenya Ayanda, Patzke Nina, Manger Paul R, Herculano-Houzel Suzana
School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, Republic of South Africa.
Brain Behav Evol. 2016;87(1):19-38. doi: 10.1159/000443201. Epub 2016 Feb 26.
It is generally believed that animals with larger bodies require larger brains, composed of more neurons. Across mammalian species, there is a correlation between body mass and the number of brain neurons, albeit with low allometric exponents. If larger bodies imperatively require more neurons to operate them, then such an increase in the number of neurons should be detected across individuals of a continuously growing species, such as the Nile crocodile. In the current study we use the isotropic fractionator method of cell counting to determine how the number of neurons and non-neurons in 6 specific brain regions and the spinal cord change with increasing body mass in the Nile crocodile. The central nervous system (CNS) structures examined all increase in mass as a function of body mass, with allometric exponents of around 0.2, except for the spinal cord, which increases with an exponent of 0.6. We find that numbers of non-neurons increase slowly, but significantly, in all CNS structures, scaling as a function of body mass with exponents ranging between 0.1 and 0.3. In contrast, numbers of neurons scale with body mass in the spinal cord, olfactory bulb, cerebellum and telencephalon, with exponents of between 0.08 and 0.20, but not in the brainstem and diencephalon, the brain structures that receive inputs and send outputs to the growing body. Densities of both neurons and non-neurons decrease with increasing body mass. These results indicate that increasing body mass with growth in the Nile crocodile is associated with a general addition of non-neurons and increasing cell size throughout CNS structures, but is only associated with an addition of neurons in some structures (and at very small rates) and not in those brain structures directly connected to the body. Larger bodies thus do not imperatively require more neurons to operate them.
人们普遍认为，体型较大的动物需要更大的大脑，而大脑由更多的神经元组成。在哺乳动物物种中，体重与脑神经元数量之间存在相关性，尽管异速生长指数较低。如果更大的体型必然需要更多的神经元来运作，那么在一个持续生长的物种（如尼罗鳄）的个体中，应该能检测到神经元数量的增加。在本研究中，我们使用细胞计数的等体积分离法来确定尼罗鳄6个特定脑区和脊髓中神经元和非神经元的数量如何随体重增加而变化。所检查的中枢神经系统（CNS）结构的质量均随体重增加而增加，异速生长指数约为0.2，但脊髓除外，其增加指数为0.6。我们发现，所有CNS结构中的非神经元数量缓慢但显著增加，其作为体重的函数缩放，指数范围在0.1至0.3之间。相比之下，脊髓、嗅球、小脑和端脑中的神经元数量随体重缩放，指数在0.08至0.20之间，但在脑干和间脑中则不然，脑干和间脑是接收输入并向生长中的身体发送输出的脑结构。神经元和非神经元的密度均随体重增加而降低。这些结果表明，尼罗鳄随生长而增加的体重与整个CNS结构中非神经元的普遍增加和细胞大小的增加有关，但仅与某些结构中神经元的增加（且速率非常小）有关，而与直接连接身体的脑结构无关。因此，更大的体型并不必然需要更多的神经元来运作。
